In commonly owned U.S. Pat. No. 3,660,771 there has been disclosed a demodulator of this type including a pulse counter which is stepped by clock pulses of a cadence substantially exceeding the higher keying frequency, the counter being reset by signal pulses derived from the incoming wave with the aid of a zero-crossing detector. With a signal pulse generated at each zero crossing, the counter has a storage capacity not less than and preferably equal to a number of clock pulses occurring during a half-cycle of the higher keying frequency so as to be reset when the incoming oscillation is of that frequency but to emit an output pulse when that frequency is lower, i.e. when the other keying frequency is being received. The output pulse of the counter sets a flip-flop which is reset simultaneously with the counter by the same signal pulse. The pulse sequence provided in this instance by the flip-flop is integrated in a low-pass filter which works into a threshold circuit so as to generate a binary voltage varying between zero and a predetermined level in conformity with the alternations of the input oscillations between the higher and the lower keying frequency.
Such a demodulator is very useful as a frequency discriminator in data transmission and voice-frequency telegraphy since it allows the use of keying frequencies differing by as little as 15% of the magnitude of the lower frequency. For the sharpest possible discrimination between two closely adjoining frequencies it is necessary to hold the storage capacity close to the number of clock cycles per half-cycle of the higher keying frequency; since the source of clock pulses is not synchronized with the incoming oscillation, that number may vary by one pulse which introduces a certain uncertainty into the parameter to be measured, i.e. the length of a half-cycle of the higher keying frequency. To minimize this uncertainty it is desirable to make the clock-pulse cadence very large in comparison with the keying frequency. With these close pulse spacings, however, it may happen that delays in pulse transmission retard the setting (or resetting) of the flip-flop in a manner resulting in an incorrect identification of the incoming frequency.